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4♣ On the previous round (assuming it was available as a natural bid) I prefer 4♣. Partner may be 4153: Opponents probably have 9 card ♠, and a singleton ♥ is also likely, given that partner has made a penalty double of a partscore. So I am hoping for a ♣ fit, and that partner doesn't think I am pulling from fear. 6♣ might be the right spot, and 5♣ will often make.

Bid ♠ at the 3 level, and X at the 4 level, like most people. Bidding ♠ at the 4 level is too committal, but it is not ridiculous, and could work. Likewise, doubling at the 3 level is likely to miss a ♠ fit, but it too is not crazy and could work. But passing in the direct seat, with a singleton ♥ and such a good hand is losing bridge.